Smart materials

Smart materials

Can materials be smart? Can they remember things? The answer is “yes!”

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Sebastian Breisch ABB Process Automation, Corporate Research Ladenburg, Germany,

The term “smart” most likely comes from the fact that such materials are by no means rigid and solid, like conventional materials, but can adapt and change form depending on external stimuli such as temperature or magnetic field. Some even have a “memory” to remember a shape that was trained before. In practice, these materials can be used for actuation, sensing and energy harvesting without any modification of the material itself.

Four classes of smart material are close to, or are already in, industrial application:
• Piezoelectric materials
• Thermal shape memory alloys (SMAs)
• Magnetic shape memory alloys (MSMAs)
• Dielectric elastomers (DEs)

Of these, the piezoelectric materials are the most mature. They are already used in industrial applications, especially in the automotive industry, which might be one reason for their ubiquity. A typical application would be in an injector for common rail engines. As sensors, they are used for force sensors and load cells, for example.

SMAs are also a mature technology. The most prominent applications of this material class are found in medicine (eg, stents). The best-known SMA is a nickel-titanium alloy paper clip that can be deformed drastically, resetting to its initial shape when heated by a candle or put into hot water. This behavior derives from the materiel’s two different crystal structures, which are temperature-dependent. At ambient temperature, the material is in its “cold” crystal structure. The SMA’s “warm” crystal structure must be trained by temperature cycling thousands of times. When, subsequently, mechanically deformed and then heated above the transition temperature at which the crystal re-orients its internal structure, the material “remembers” its trained shape, to which it reverts. The transition temperature for standard materials is around 60 °C.

01 Applying an external magnetic field (B) leads to a proportional deflection of the folded crystal structure. In standard applications, resetting to the original shape is done via an external force (F).
01 Applying an external magnetic field (B) leads to a proportional deflection of the folded crystal structure. In standard applications, resetting to the original shape is done via an external force (F).

For industrial SMA actuators, a standard design is a simple wire that can be stretched and is then pulled back when heated (by passing a current, for example). This structural change can exert high forces if a large cross-section is used.

Similar to the SMAs are the MSMAs, which react not only to temperature, but also to magnetic fields. MSMA manufacture is challenging because casting the required monocrystalline ingot is a complex process. Within this ingot, a magnetic polarized and folded magnetic crystal structure will evolve during solidification. The MSMA elements (“sticks”) are cut out of the ingot in a beneficial orientation. The folded, magnetically polarized crystal structure within these sticks allows them to deform when an external magnetic field is applied →01.

The combination of thermal and magnetic effects makes MSMAs ideal for applications where both a thermal and a magnetic response is required – eg, in a domestic miniature circuit breaker (MCB).

DEs form the last class of smart materials listed above. The rubbery DE material is typically sandwiched between two electrode plates of opposite polarity, as in a standard capacitor. An applied voltage causes the plates to mutually attract, squeezing the elastomer. This basic deformation allows a wide versatility of actuator designs. In sensing mode, any displacement of the DE (configured as a membrane, for example) changes the capacitance, allowing precise deformation measurement. The first industrial products employing this principle will be launched soon.

Smart materials offer a wide range of functionality for many actuation and sensing applications – and all with a very simple design with a low part count compared to alternative solutions. Smart, indeed. 


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